Electrolytic method of producing aluminum



- reduced-to about 750 to 950 2,742,414 latented Apr. 17,1956

United States Patentfi '.2,742,414 ELECTROLYTIC METHOD OF PRODUCING ALUMINUM v The present invention relates to an electrolyticmethod producing aluminum and particularly to a method of at a low temperature in fusing that electrolysis is carried out of. melting an electrolytic bath electrolysis of aluminum so atalow temperature. I

,Ihe object of the present invention is to provide a method whereby an electrolytic bath is melted at alow temperature ..in the .fusing electrolysis of aluminum, at;the sametime .thesolubility of alumina in the electrolytic bath canbe increased, the electrolysis temperature can there fore. be maintained low, and the damage ofthe electrolytic cell. can "be made small. v

.tAtthe. present, most commonly...and widelylcnown method of manufacturing aluminum by fusingelectrolysis is totadd 1010 .15 parts of aluminate 85 .to 90 p.ar.ts ..of molten cryolite and to electrolyze the fused mixturezat a ttemperattue .ofzabout-950 to 1000 C. However, .keeping the electrolytic bath at such. a high temperaturehas various disadvantages such as acceleration of damage of :the 'cell, diifi'cul-ty in operation, etc. Therefore, said methodwis not entirely satisfactory.

= I have-found that, by using sodium borofluoride and/ or aluminum-sodium borofluoride together with cryoliteinstead of. cryolite alone in the conventional method, the solubility of alumina can be increased, at the same time the melting temperature of the electrolytic. bat-hcanbe C; and. therefore the electrolysis can be-etfected smoothly atatemperatu-re about C. lower than :in the conventional method.

Aluminum-sodium' borofluoride is,:1:as :far was? ;I ;know, first manufactured by me. Aluminum-sodium borofluoride is so stable :as not to decompose even atacon'sider: ably high temperature. It melts completely at a comparatively 'low' temperature of about 68040 720 C. The solubility of. alumina aluminum-sodium borofluoride. is .high. I have found that 30 parts, of aluminajcan be complet'ely'dissolved and melted in 100 parts of 'said borofluoride at about".720'C.

. Thermanufacture of said borofluorideis not the'purpose of the present invention and therefore details thereof are omitted. In general, said borofluoride is prepared as follows:

A solution of sodium aluminate or a mixed solution of sodium aluminate and sodium hydroxide (A1NaOzNaOH or ZAINaOz-NaOH) is added to fluoboric acid. The reaction proceeds as shown in the following formulas and the aluminumsodium borofluoride is formed as a white precipitate, said precipitate being then fractionated and dried:

(3) 3HBF4+2AlNaOz NaOH- Al2Na3Os BF4) 3+2H2O For example, 1000 cc. of 1 mole solution of sodium aluminate combined with caustic soda (ZAINaOz-NaOH) were gradually added with stirring to 650 cc. of 33.5%

' those in the existing methods.

flnoboric acid solution. The reaction proceeded anddiffic'ulty soluble aluminum-sodium"bmofluoride was formed asa'white precipitate. The whole was allowed to stand for one day and :night' to. comp'letethe. reaction. The precipitate was then filtered, dr'ied'at1'10C., and was further roasted at 5505102600 C. for 3 hours. By this way, 430 grams of the-borofluoride having amelting point of 680 to 720.C.- were obtained.

in carrying out the method or the present invention, in general, -if the amount of sodium borofluoride. andfor aluminum-sodium borofluoride relative to .cryolite is @increasedthe melting temperature will. be. reduced. while .the solubility of alumina will 'be increased. However, the use of excess amount ofvsodium borofluorideand/ or aluminum-sodium borofluoride is. undesirable because it tends to cause the so-called anodeefiiectinthe. electrolysis, to make the electrolysis diflicult and to corrode the electrolytic cell and electrodes. "For example, when a mixture .of. grams of 1 aluminum-sodium. borofluoride, .100-gramsof-cryolite and lflilggrarnsaof alumina, is-heated in anelectric furnace, rcornpletel-y melts at a low. temper-ature,;namely' .7l0iC. However, for the-above mentioned reasons, this composition is not satisfactoryv as an electrolytic bath. Qn :the other hand, when theamount ofthense: of sodium..borofluoride:and/or. aluminumsodium. borofluoride. .is' too -.small.: satisfactory reduction :of the-meltingternperature :andincrease in the solubility of alumina will znot; be attained. 1

I have found that the composition of 15 to 30% by weight of alumina, 50 to"80%' by weight of cryolite and 5. to-% why-weight. of sodium 'borofiuor-ide'and/or aluminum-sodium" borofluoride is-satisfactory in practice. That is to :say, a mixture having the compositionwvithin *this rang'e comp letelymeltsat about 75010900" C. and its electrolysis I proceeds smoothly and elficiently.

T-he melting and electrolysis operations and equipments to be used "therefor are nothing particularly different from --For example, an electrolytic bath mixture having .the composition within the above mentioned range is put into an electrolytic cell which may be a crucible made of graphite and constitutes a cathode in case of electroiys'is andlisfused by' a proper "heating means {such as electric furn'ace. [Any-known anode. such as a graphite electrode-is immersedin said fused electrolyte and the electric current is fed "thereto-for electrolysis. As the eiectrolysis' proceeds sodium borofiuorideor aluminum-sodium borofluoride be decomposed and'lost 'little by iittle. "Therefore, particularly whenthe electrolysisis carried out on a continuous'basis a suitable amount of the compound must be properly added so that the electrolytic bath may maintain substantially the above mentionedrange of composition. .Similarly aluminnm fluoridejn .cryoliteis decomposed and there is formed .orseparat ed sodiuni fluoride, ,is .accumulated astthe electrolysis proceeds. Therefore, 'it is necessary to suitably add aluminum fluoride from time to time to reproduce cryolite and to keep the composition within the range mentioned.

Some examples of the present invention are given below for a more complete understanding of the invention. However, the invention is not to be limited to these examples.

Example 1 grams of alumina, 172 grams of aluminum-sodium borofluoride and 625 grams of cryolite as an electrolyte was charged into a crucible which is made of graphite and is about 10 cm. in inside diameter and about 13 cm. in depth. Heat is applied thereto by means of an electric furnace and the mixture melted at 840 to 880 C. and came to maintain desired fluidity. A bar-shaped graphite anode of about 2 cm. in diameter was immersed in the fused electrolyte in said electrolytic cell which is to constitute a cathode and the electric current was turned on to place the cell in electrolysis opera tion. The electrolysis proceeded under said temperature. The voltage ranged from 11 to 13 volts and the current density at the anode was 0.8 ampere/cml About 6 hours later, deposition of about 51 grams aluminum was recognized in the bottom of the crucible.

Example 2 A mixture of 153 grams of alumina, 129 grams of aluminum-sodium borofiuoride and 718 grams of cryolite was put into a crucible (electrolytic cell) similar to that of Example 1 and was heated by an electric furnace. The mixture completely melted at 810 to 840 C. and came to maintain the desired fluidity. The electrolysis proceeded at the voltage of 10 to 13 volts and the current density (at the anode) of 0.7 ampere/cm. for 7 hours in the same manner as in Example 1. About 56 grams of aluminum were deposited thereby.

Example 3 A mixture of 152 grams of powdered alumina, 42 grams of aluminum-sodium borofluoride and 806 grams of cryolite was put into a crucible (electrolytic cell) similar to that of Example 1 and was heated by an electric furnace. The mixture completely melted at 860 to 890 C. and came to maintain the desired fluidity. When the electrolysis was proceeded at the voltage-of 10 to 14 volts and the current density (at the anode) of 0.7 ampere/cm. in the same manner as in Example 1, deposition of about 5 3 grams aluminum was recognized in 6.5 hours.

Example 4 A mixture of 150 grams of powdered alumina, 150 grams of aluminum-sodium borofluoride and 700 grams of cryolite was heated in an electrolytic cell similarly to the preceding examples. The mixture completely melted at 760-800" C. and came to maintain the desired fluidity. The electrolysis proceeded under substantially the same condition as in Example 3 and deposition of about 62 grams of aluminum was recognized in 7.5 hours.

ExampleS Example 6 A mixture of 18 0 grams of powdered alumina, 190 grams of sodium borofluoride and 630 grams of cryolite was heated in an electrolytic cell similar to that of Example l.

The mixture melted completely at 830 to 880 C. and came to maintain the desired fluidity. The electrolysis proceeded at the voltage ranging from 11 to 14 volts and the current density (at the anode) of 0.8 ampere/cm? in the same manner as in Example 1 and after 6 hours deposition of about 45 grams of aluminum was recognized.

Example 7 A mixture of 150 grams of powdered alumina, 140 grams of sodium bore-fluoride and 710 grams of cryolite was put into an electrolytic cell similar to that of Example land was heated by an electric furnace. The mixture completely melted at 820 to 860 C. and came to maintain the desired fluidity. The electrolysis proceeded at the voltage ranging from 11 to 13 volts and the current density (at the anode) of 0.7 ampere/cm. in the same way as in Example 1 and after 8 hours about 68 grams of aluminum were deposited.

It will be noted that the voltage in the above examples is somewhat higher than in the conventional method. However, this is caused by the fact that these specific examples are made on a small scale so that there are comparatively high contact resistance and other resistances. There will be no substantial difference of voltage if carried out on a large scale.

The capacity of the graphite crucible or electrolytic cell used in the above examples is small and accordingly temperature drop due to radiant heat loss is comparatively large. Therefore, in these examples, the electric current must be passed to the electric furnace at proper time in the electrolysis operation to maintain the desired fluidity. However, there would be no such necessity in case the electrolysis is carried out on a large scale because once the mixture is melted the temperature and fluidity can be maintained satisfactorily by the joule heat produced in the course of the electrolysis.

I claim:

1. A method of preparing aluminum by electrolysis which comprises electrolyzing at a temperature from 750 to 950 C. a fused electrolyte consisting of 15 to 20% by weight of alumina, to by weight of cryolite and 5 to 20% by weight of a substance selected from the group consisting of sodium borofluoride, aluminum-sodium borofluoride and their mixture.

2. A method of preparing aluminum by electrolysis which comprises electrolyzing a fused electrolyte consisting of 15 to 20% by weight of alumina, 60 to 80% by weight of cryolite and 5 to 20% by weight of a substance selected from the group consisting of sodium borofluoride, aluminum-sodium borofluoride and their mixture.

References Cited in the file of this patent UNITED STATES PATENTS 244,234 Higgs July 12, 1881 FOREIGN PATENTS H 648,337 Great Britain Ian. 3, 1 951 

2. A METHOD OF PREPARING ALUMINUM BY ELECTROLYSIS WHICH COMPRISES ELECTROLYZING A FUSED ELECTROLYTE CONSISTING OF 15 TO 20% BY WEIGHT OF ALUMINA, 60 TO 80% BY WEIGHT OF CRYOLITE AND 5 TO 20% BY WEIGHT OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF SODIUM BOROFLUORIDE, ALUMINUM-SODIUM BOROFLUORIDE AND THEIR MIXTURE. 